An air bag device is widely provided in a vehicle as a safety device for occupant protection that protects an occupant from impact when a vehicle undergoes collision. In recent years, an air bag device needs to be compact and light in weight, and accordingly, the size of an inflator has been reduced. As a result, the gas production amount decreases, and thus a highly-airtight air bag with little loss of gas is required.
In order to meet such a demand, it is important to suppress gas leakage from the surface of an air bag. Fabric made from synthetic fibers is mainly used as the material of an air bag, and there are gaps to some extent between threads that constitute the fabric. Furthermore, when gas is supplied from the inflator and the pressure inside the air bag increases, there is a possibility that these gaps will enlarge and as a result, and gas will tend to leak.
Also, it is known that the output of the inflator increases by being heated, and the output thereof increases due to the inflator being placed in a high temperature environment such as summer after the inflator is provided in a vehicle. Furthermore, there is also a concern that the load on the air bag increases due to an increase in the output, causing an accident such as burst, and thus a sufficient strength is also required.
For example, Patent Literature 1 discloses a method by which a fabric with gas permeability suitable for an air bag is obtained by controlling elongation in response to stress of the raw yarn that constitutes the fabric. However, the physical properties change depending on the weaving process and thermal history and stress history in the working process, and thus even though the elongation in response to stress of the raw yarn is controlled, it cannot be said that the elongation is similar in the fabric state, and control is not sufficient.
Also, Patent Literature 2 discloses a method in which elongation under a stress of 300 N/cm is set to a warp/weft total of 40% or more in order to reduce the load applied to stitches and prevent gas leakage. However, gaps between threads of an easily-stretchable fabric enlarge easily, and there is a risk that gas leakage from portions other than stitches will increase.